During conventional semiconductor manufacturing, a lightly-doped drain (LDD) spacer is formed by blanket deposition and blanket etchback (spacer etch) of the silicon oxide material. Unfortunately, the fluorocarbon gases (such as CF.sub.4 and CHF.sub.3) used for spacer etching leave carbon and fluorine embedded in the silicon surface. These residues impair the subsequent formation of TiSi.sub.x by preventing deposited titanium from reacting with silicon on the surface. TiSi.sub.x formed on a contaminated silicon surface will have a higher sheet resistance than TiSi.sub.x formed on clean silicon. U.S. Pat. No. 5,895,245 (Harvey et. al.) discloses the use of a CF.sub.4 /H.sub.2 O plasma to remove the carbon and fluorine residues left after spacer etch.
More recently, spacer etching has been done using a photoresist mask to prevent removal of oxide from certain regions of the wafer. The purpose of this masked spacer etch is to prevent TiSi.sub.x formation at those locations, thereby creating sections of the gate electrode with high resistivity. High resistivity in certain controlled locations can be advantageous to device designers; however, in general, the lowest possible resistivity is desired to make the device as fast as possible.
The photoresist mask must be removed after the spacer etch. Conventionally, this is done with an O.sub.2 plasma. More specifically, in the prior art, process steps are performed in the following order. First, a masked spacer etch is performed. Next, an O.sub.2 plasma ash is performed to remove the photoresist mask. The use of an O.sub.2 plasma forms a layer of oxide on the wafer, covering the contaminants. After the O.sub.2 plasma ash is performed, prior art processes treat the silicon surface with a CF.sub.4 /H.sub.2 O plasma to remove contaminants. Following the aforementioned steps, titanium deposition and TiSi.sub.x formation processes are performed.
Unfortunately, the conventional process results in poor TiSi.sub.x formation and increased sheet resistance. Particularly, prior art processes tend to leave carbon and fluorine residues under the oxide formed by the O.sub.2 plasma ash. Consequently, the subsequent CF.sub.4 /H.sub.2 O process is prevented from removing the carbon and fluorine residues. These residues then impede the efficient formation of a silicide layer.
Thus, a need exists for a formation method which results in the lowest possible resistivity in certain desired regions of the semiconductor substrate. Yet another need exists for a formation method in which contaminants are not trapped on the surface of the semiconductor substrate and thus prevented from being removed.